Field of the Invention and Related Art Statement
The present invention relates to an apparatus for supporting an objective lens movably in two substantially orthogonal directions, and more particularly to an apparatus for supporting an objective lens provided in an optical pick-up movably in a focusing direction parallel to an optical axis of the objective lens and a tracking direction perpendicular both to the optical axis of the objective lens and to a tangential direction along which a signal track is formed on an optical disc such as compact disc and video disc and opto-magnetic disc.
Such an objective lens supporting apparatus has been widely used in the optical pick-up in various kinds of optical information processing apparatuses such as compact disc player, video disc player and opto-magnetic data recording and reproducing apparatus. In the known optical pick-up, a recording light beam is made incident upon a record medium by means of the objective lens in a focused condition, and a light flux reflected by the record medium is detected by a photodetector via the objective lens. In this case, in order to reproduce the information signal precisely, a light spot formed on the record medium should trace or follow the information track in an accurate manner. To this end, there are provided a focusing servo control mechanism for moving the light spot in the direction of the optical axis of the objective lens and a tracking servo control mechanism for shifting the light spot in the tracking direction.
There has been proposed as optical pick-up for effecting the above mentioned focusing and tracking controls by moving the objective lens in the focusing and tracking directions in accordance with focusing and tracking errors, respectively. For instance, in an U.S. Pat. No. 4,646,283 issued to M. Ito et al on Feb. 24, 1987, there is disclosed an optical pick-up in which a lens holder having the objective lens secured thereto is supported by means of four resilient wires movably in the focusing and tracking directions. FIG. 1 is a perspective view illustrating this known pick-up including the four resilient wires. The pick-up comprises an objective lens 1 which is secured to a lens holder 2, a stationary member 3 secured to a base member 4, and four resilient wires 5. The resilient wires 5 may be formed by, for instance, piano wires. The resilient wires 5 are arranged in parallel with each other and are extended in a tangential direction Y which is parallel to the direction of the information track formed on the optical record disc not shown. Therefore, the lens holder 2 and thus the objective lens 1 can be moved both in the focusing direction Z and in the tracking direction X. In the apparatus shown in FIG. 1, there are also provided moving coils secured to the lens holder 2 and permanent magnets for generating magnetic fluxes passing through the moving coils to produce forces for driving the lens holder 2 in the focusing and tracking directions. Since such an electro-magnetic actuator has been well known in the art and is not concerned with the subject matter of the present invention, its detailed construction and function are omitted here.
In the known optical pick-up apparatus shown in FIG. 1, the lens holder 2 is liable to rotate about the tangential axis Y and the optical axis of objective lens 1 is inclined, when the lens holder is moved in the tracking direction X. This is due to the fact that the spring constant for the tilting motion about the tangential direction Y which relates to the resistance against the bending motion of the wires is substantially smaller than that about the tracking direction X which relates to the resistance against the shrinkage and extension of the wires. Further, since the four resilient wires 5 are arranged in parallel with each other, the lens holder 2 is liable to be vibrated at undesired resonance frequency. Moreover, the lens holder 2 is liable to be affected by external force to a great extent. Due to the above mentioned undesired phenomena, the focusing and tracking servo controls could not be carried out precisely, and thus the information signal could not be read out or recorded accurately.
In order to avoid the above mentioned drawbacks of the known optical pick-up shown in FIG. 1 in which the four resilient wires are arranged in parallel with each other in the tangential direction Y, there has been proposed another known optical pick-up in which four resilient wires are arranged in a non-parallel manner. Such a known optical pick-up is described in Japanese Patent Application Publication Kokai-Sho 60-197,942. FIG. 2 is a perspective view showing this known optical pick-up apparatus. In FIG. 2 portions similar to those of FIG. 1 are denoted by the same reference numerals used in FIG. 1. An objective lens 1 is supported by a lens holder 2 and the lens holder is supported by a stationary members 3a and 3b by means of four resilient wires 5a, 5b, 5c and 5d. As illustrated in FIG. 2, the two wires 5a and 5b are arranged in parallel with each other and the two wires 5c and 5d are arranged in parallel with each other, but the wires 5a, 5b are not in parallel with the wires 5c, 5d, respectively viewed in the focusing direction Z. That is to say, extensions of the wires 5a and 5c make an angle substantially equal to 90.degree. and the wires 5b and 5d make the same angle. It should be noted that the wires 5a and 5c situate in the same plane viewed in the plane perpendicular to the focusing direction Z and, similarly the wires 5b and 5d are existent in the same plane perpendicular to the focusing direction Z. The known optical pick-up depicted in FIG. 2, the lens holder 2 is hardly tilted about the tangential direction Y and is hardly influenced by the external force, so that the focusing and tracking servo controls can be performed in an accurate manner.
In the data processing apparatus using the optical record disc, particularly the opto-magnetic disc, it is required to utilize the disc at a high efficiency to make a data record area as wide as possible. Therefore, it is advantageous to record the data signal up to a region as close as possible to a center of the disc. To this end, a turntable on which the record disc is placed and is rotated is designed to have a diameter as small as possible, so that the optical pick-up can be moved toward the center of the disc as close as possible. However, in the known optical pick-up shown in FIG. 2, since the wires 5a to 5d are not arranged in parallel with each other, the space which the wires occupy becomes large and thus the pick-up could not be moved much closer to the center of the record disc, otherwise the wires would interfere with the turntable. Therefore, the utilization efficiency of the record disc would be decreased and an amount of data which can be recorded on the record disc is limited.
Further, in many optical pick-ups, there is provided a device for detecting the position of the objective lens in the tracking direction. In this device, a light shielding member or plate is secured to the lens holder and a light source and a light detector are arranged on the base member such that an amount of the light flux emitted from the light source toward the light detector and being shielded by the shielding plate is changed in accordance with the position of the lens holder relative to the base member. Usually the shielding plate is secured to the lens holder near a portion at which the resilient wire is secured to the lens holder, so that the dimension of the optical pick-up viewed in the tracking direction X is liable to be large, because the shielding plate has to be arranged not to interfere with the resilient wire.